Ballast with lamp-to-earth-ground fault protection circuit

Abstract

A ballast ( 100 ) includes an inverter ( 140,144,146 ) and a protection circuit that prevents excessive lamp-to-earth-ground fault current. The protection circuit includes a transformer ( 202,204,206,208,210 ) and an inverter disable circuit ( 300 ). The transformer measures a first current going out of one set of ballast output terminals ( 106,108 ) and a second current going into another set of ballast output terminals ( 206,208 ). In response to a substantial imbalance between the first current and the second current, inverter disable circuit ( 300 ) terminates inverter switching. Preferably, protection circuit further includes a restart timer circuit ( 400 ) that, following termination of inverter switching in response to a fault condition, prevents the inverter from restarting for a predetermined delay period.

What is claimed is:  
     
       1. A ballast for powering at least one gas discharge lamp, comprising; 
       first, second, third, and fourth output connections adapted for connection to the gas discharge lamp, wherein the first and second output connections are adapted for connection to a first filament of the lamp, and the third and fourth output connections are adapted for connection to a second filament of the lamp;  
       an inverter for supplying a high frequency alternating current to the gas discharge lamp, the inverter comprising:  
       an inverter drive circuit having a voltage supply input for receiving a supply voltage, the inverter drive circuit being operable to: (i) provide inverter switching as long as the supply voltage is greater than a predetermined shutdown voltage; and (ii) cease to provide inverter switching when the supply voltage falls below the predetermined shutdown voltage; and  
       a bootstrap power source that is operable, while inverter switching is occurring, to provide power to the inverter drive circuit; and  
       a protection circuit, comprising:  
       a transformer, comprising:  
       a first primary winding coupled in series with the first output connection;  
       a second primary winding coupled in series with the second output connection;  
       a third primary winding coupled in series with the third output connection;  
       a fourth primary winding coupled in series with the fourth output connection; and  
       a secondary winding;  
       an inverter disable circuit, comprising:  
       a disable output coupled to the voltage supply input of the inverter drive circuit;  
       a transistor having a base, a collector, and an emitter, wherein the emitter is coupled to circuit ground;  
       the secondary winding of the transformer, the secondary winding being coupled between a first node and circuit ground;  
       a first resistor coupled between the first node and circuit ground;  
       a diode coupled between the first node and the base of the transistor;  
       a capacitor coupled between the base of the transistor and circuit ground;  
       a second resistor coupled between the base of the transistor and circuit ground; and  
       a third resistor coupled between the disable output and the collector of the transistor; and  
       a restart timer circuit, comprising:  
       a restart input coupled to the bootstrap power source of the inverter;  
       a restart output coupled to the voltage supply input of the inverter drive circuit;  
       a transistor having a collector, an emitter, and a base, wherein the emitter is coupled to circuit ground;  
       a series combination of a diode and a first resistor coupled between the restart input and a second node;  
       a capacitor coupled between the second node and circuit ground;  
       a second resistor coupled between the second node and the base of the transistor;  
       a third resistor coupled between the base of the transistor and circuit ground; and  
       a fourth resistor coupled between the restart output and the collector of the transistor.  
     
     
       2. A ballast for powering at least one gas discharge lamp, comprising: 
       an inverter for supplying a high frequency alternating current to the gas discharge lamp, the inverter including an inverter drive circuit having a voltage supply input for receiving a supply voltage, the inverter drive circuit being operable to: (i) provide inverter switching as long as the supply voltage is greater than a predetermined shutdown voltage; and (ii) cease to provide inverter switching when the supply voltage falls below the predetermined shutdown voltage;  
       first, second, third, and fourth output connections adapted for connection to the gas discharge lamp, wherein the first and second output connections are adapted for connection to a first filament of the lamp, and the third and fourth output connections are adapted for connection to a second filament of the lamp; and  
       a protection circuit coupled to the inverter and the first, second, third, and fourth output connections, the protection circuit comprising:  
       a transformer comprising:  
       a first primary winding coupled in series with the first output connection;  
       a second primary winding coupled in series with the second output connection;  
       a third primary winding coupled in series with the third output connection;  
       a fourth primary winding coupled in series with the fourth output connection; and  
       a secondary winding operably coupled to the inverter;  
       an inverter disable circuit chat includes the secondary winding of the transformer and that is coupled to the voltage supply input of the inverter drive circuit, the inverter disable circuit being operable, in response to a nonzero voltage across the secondary winding of the transformer, to terminate inverter switching by coupling the voltage supply input to circuit ground; and  
       a restart tuner circuit coupled to the inverter, the restart timer circuit being operable, following termination of inverter switching, so prevent the inverter from resuming inverter switching for at least a predetermined restart period.  
     
     
       3. The ballast of  claim 2 , wherein the inverter disable circuit comprises: 
       a disable output coupled to the voltage supply input of the inverter drive circuit;  
       a transistor having a base, a collector, and an emitter, wherein the emitter as coupled to circuit ground;  
       the secondary winding of the transformer, the secondary winding being coupled between a first node and circuit ground;  
       a first resistor coupled between the first node and circuit ground;  
       a diode coupled between the first node and the base of the transistor;  
       a capacitor coupled between the base of the transistor and circuit ground;  
       a second resistor coupled between the base of the transistor and circuit ground; and  
       a third resistor coupled between the disable output and the collector of the transistor.  
     
     
       4. The ballast of  claim 2 , wherein: 
       the inverter further comprises a bootstrap power source that is operable, while inverter switching is occurring, to provide power to the inverter drive circuit and the restart timer circuit; and  
       the restart timer circuit comprises:  
       a restart input coupled to the bootstrap power source of the inverter;  
       a restart output coupled to the voltage supply input of the inverter drive circuit;  
       a transistor having a collector, an emitter, and a base, wherein the emitter is coupled to circuit ground;  
       a series combination of a diode and a first resistor coupled between the restart input and a second node;  
       a capacitor coupled between the second node and circuit ground;  
       a second resistor coupled between the second node and the base of the transistor;  
       a third resistor coupled between the base of the transistor and circuit ground; and  
       a fourth resistor coupled between the restart output and the collector of the transistor.  
     
     
       5. A ballast for powering at least one gas discharge lamp, comprising: 
       an inverter for supplying a high frequency alternating current to the gas discharge lamp;  
       first, second, third, and fourth output connections adapted for connection to the gas discharge lamp, wherein the first and second output connections are adapted for connection to a first filament of the lamp, and the third and fourth output connections are adapted for connection to a second filament of the lamp; and  
       a protection circuit coupled to the inverter and the first, second, third, and fourth output connections, the protection circuit comprising:  
       a transformer, comprising:  
       a first primary winding coupled in series with the first output connection;  
       a second primary winding coupled in series with the second output connection;  
       a third primary winding coupled in series with the third output connection;  
       a fourth primary winding coupled in series with the fourth output connection; and  
       a secondary winding operably coupled to the inverter, the secondary winding having a voltage that is: (i) substantially zero in the absence of a lamp-to-earth-ground fault condition; and (ii) nonzero in the presence of a lamp-to-earth-ground fault condition.  
     
     
       6. The ballast of  claim 5 , wherein the first, second, third, and fourth primary windings have the same number of wire turns. 
     
     
       7. The ballast of  claim 6 , wherein the secondary winding has a number of wire turns that is substantially greater than the number of wire turns on the first, second, third, and fourth primary windings. 
     
     
       8. The ballast of  claim 5 , wherein the protection circuit comprises a transformer having: 
       a first primary winding coupled in series with the first output connection;  
       a second primary winding coupled in series with the second output connection;  
       a third primary winding coupled in series with the third output connection;  
       a fourth primary winding coupled in series with the fourth output connection; and  
       a secondary winding operably coupled to the inverter, the secondary winding having a voltage that is: (i) substantially zero when the first current is substantially equal to the second current; and (ii) nonzero when the first current is no; substantially equal to the second current.  
     
     
       9. The ballast of  claim 8 , wherein: 
       the inverter includes an inverter drive circuit having at voltage supply input for receiving a supply voltage, the inverter drive circuit being operable to: (i) provide inverter switching as long as the supply voltage is greater than a predetermined shutdown voltage; and (ii) cease to provide inverter switching when the supply voltage falls below the predetermined shutdown voltage; and  
       the protection circuit further includes an inverter disable circuit that includes the secondary winding of the transformer and that is coupled to the voltage supply input of the inverter drive circuit, the inverter disable circuit being operable, in response to a nonzero voltage across the secondary winding of the transformer, to terminate inverter switching by coupling the voltage supply input to circuit ground.  
     
     
       10. The ballast of  claim 9 , wherein the inverter disable circuit comprises: 
       a disable output coupled to the voltage supply input of the inverter drive circuit;  
       a transistor having a base, a collector, and an emitter, wherein the emitter is coupled to circuit ground;  
       the secondary winding of the transformer, the secondary winding being coupled between a first node and circuit ground;  
       a first resistor coupled between the first node and circuit ground;  
       a diode coupled between the first node and the base of the transistor;  
       a capacitor coupled between the base of the transistor and circuit ground;  
       a second resistor coupled between the base of the transistor and circuit ground; and  
       a third resistor coupled between the disable output and the collector of the transistor.  
     
     
       11. The ballast of  claim 9 , wherein the protection circuit further comprises a restart timer circuit coupled to the inverter, the restart timer circuit being operable, following termination of inverter switching, to prevent the inverter from resuming inverter switching for at least a predetermined restart period. 
     
     
       12. The ballast of  claim 11 , wherein: 
       the inverter further comprises a bootstrap power source that is operable, while inverter switching is occurring, to provide power to the inverter drive circuit and the restart timer circuit; and  
       the restart timer circuit comprises:  
       a restart input coupled to the bootstrap power source of the inverter;  
       a restart output coupled to the voltage supply input of the inverter drive circuit;  
       a transistor having a collector, an emitter, and a base, wherein the emitter is coupled to circuit ground;  
       a series combination of a diode and a first resistor coupled between the restart input and a second node;  
       a capacitor coupled between the second node and circuit ground;  
       a second resistor coupled between the second node and the base of the transistor;  
       a third resistor coupled between the base of the transistor and circuit ground; and  
       a fourth resistor coupled between the restart output and the collector of the transistor.